1. Field of the Invention
The subject invention relates to an assembly for mixing fluids (i.e., gases or liquids), and more particularly to an assembly that accurately mixes two or more high-pressure fluid sources and is adapted for use in applications, such as for example, chromatography.
2. Background of the Related Art
In many chemical or industrial processes and applications, the need to accurately mix or combine two or more fluids is a necessary and important step in the production line. For example, in a chromatographic process, the accuracy of the solvent mixture (i.e., mixture of two or more solvents) affects the precision and accuracy of the subsequent chromatographic analysis.
Chromatography is a process for separating mixtures by virtue of their differences in absorbency. FIG. 1 illustrates a typical chromatographic process. Although there are other types of chromatography (e.g. paper and thin layer), most modern applications include a mobile phase and a stationary phase and the separation of the fluid mixture takes place in a column. The column is usually a glass or metal tube of sufficient strength to withstand the pressures that may be applied across it. The column can be for example, a packed bed or open tubular column. The column contains the stationary phase of the process, i.e., the material for which the components to be separated have varying affinities. The mobile phase of the chromatographic process is comprised of a solvent mixture into which the sample to be analyzed is injected. The mobile phase enters the column and the sample is absorbed onto the stationary phase. The solvent mixture is not absorbed on the stationary phase, but passes through the column.
The materials that comprise the mobile and the stationary phase vary depending on the general type of chromatographic process being performed, i.e., gas or liquid chromatography. The mobile phase in gas chromatography is generally an inert gas. The stationary phase is generally an adsorbent or liquid distributed or the surface of a porous, inert support. The mobile phase in liquid chromatography is a liquid of low viscosity that flows through the stationary bed. This may be comprised of an immiscible liquid coated onto a porous support, a thin film of liquid phase bonded to the surface of a sorbent, or a sorbent of controlled pore size.
As illustrated in FIG. 1, a first pump is used to draw a first solvent from a tank and supply it at a desired flow velocity and pressure to a T-shaped piping connector. A second pump is used to draw a second solvent from a second tank and supply it at a desired flow velocity and pressure to the T-shaped piping connector. At the T-shaped piping connector, the solvents are blended to achieve a solvent mixture having desired properties. The flow velocity of each solvent can be adjusted over time so as to vary the composition of the solvent mixture over time. A variation in the solvent mixture over time is called a solvent or compositional gradient.
A third pump is used to supply the sample or feed to a second T-shaped piping connection where it is injected into the solvent mixture and blended therewith, forming the mobile phase. The mobile phase runs through the column typically by action of the first and second pumps whereby the sample is absorbed onto the stationary phase. As the sample flows through the column, its different components will adsorb to the stationary phase to varying degrees. Those with strong attraction to the support move more slowly than those with weak attraction and this is how the components are separated. After the sample is flushed or displaced from the stationary phase, the different components will elute from the column at different times. The components with the least affinity for the stationary phase will elute first, while those with the greatest affinity for the stationary phase will elute last. A detector analyses the emerging stream by measuring a property, which is related to concentration and characteristic of chemical composition. For example, the refractive index or ultra-violet absorbance is measured.
In high pressure, low dead volume chromatography applications, uniform solvent mixtures and precise solvent gradients are required and consequently, very precise flow streams from the solvent pumps are necessary. A solvent flow transient caused by either the first or the second pump produces a solvent gradient error that affects the precision and accuracy of the chromatographic analysis. Unfortunately, all presently available pumps have flow transients to some extent.
In view of the above, there is a need for a mixer assembly that is capable of mixing two or more fluid streams without adding significant dead volume to the system and produces a uniform combined stream that relaxes the need for very precise flow delivery from the system pumps.